Dual Z motor not staying together at rest

I'm building a large format 3D printer (400mm x 400mm bed) based on the Ultimaker mechanics that uses Z motors at the sides to lift the bed. Problem is that I've built the machine pretty free and one of the Z motors turns a bit at rest (darn gravity). On my RAMPS powered printers, connecting the two Z motors to the board results in them being locked together. Even with power off, when you turn one, the other turns. I have to unplug one to hard level the bed. This doesn't happen with this board so one side can "sag". Am I missing something in setup? Is there a way to tie them together? Do I need to wire them together then connect that to the board?

One other oddity I noticed is that when I turn up the Z axis motor current in firmware, both motors will start to lift the bed, then one will stop. If you give it a nudge, it will start again but then it will stop again or the other might do it.

The two Z motor connectors on RAMPS are wires in parallel. This means the Z driver has to supply twice as much current as normal, and that frequently results in an overheating Z driver (if the pot is set high enough to provide the current needed by the motors), or a Z axis that doesn't move smoothly (if the current is set lower so as to keep the drivers cool). On Duet the two Z motor connectors are wired in series, which works much better with most motors.

The parallel connection will lock the motors together if they are rotated quickly enough. They won't remain locked if one of them is rotated slowly.

You can connect the two motors in parallel and connect them to one of the connectors on the Duet if you wish, with 2 jumper sin the other connector. Alternatively, there are ways of getting the motors in sync, by driving them from 2 separate drivers and then either by using a separate homing switch for each motor, or by using a Z probe to adjust the motors individually to get the bed level on both sides. I'm hoping to make this a standard part of the firmware about 2 weeks from now. In the meantime, here are some links to what other users are doing:

Regarding the "oddity", this suggests that the combination of motor torque (at the current you are using) and gearing is too low for the weight of the bed and the friction in the system. What motors are they? Increasing motor current up to about 85% of the rated motor current should help. Using leadscrews with a smaller lead would increase the gearing, and also make the motors less likely to move when you turn the power off.

The Robodigg site gives minimal data on those motors, but it does say they are 1.2A motors. So you can drive two in parallel from a Duet WiFi or Duet Ethernet. I suggest a current setting of about 1.0A if you wire them in series, or 2.0A if you wire them in parallel.

Sorry, one more question. I'm thinking I need Z motors with more torque. Robotdigg sells the same setup with 60mm high torque motors. I believe they are 1.8a each. Would that still be OK for wiring them in parallel?

Update: I bought higher torque motors (Nema17 60mm 0.65N.m high torque) for the Z axis. They are now wired individually again (not connected together. They still stall individually going up if Max speed is set to over 1800 in config.g.

Update: I bought higher torque motors (Nema17 60mm 0.65N.m high torque) for the Z axis. They are now wired individually again (not connected together. They still stall individually going up if Max speed is set to over 1800 in config.g.

I have the motor current for Z set to 1000. Can it be set higher?

Setting motor current to 85% of the motors maximum rating is usually thought to be a good rule of thumb.

30mm/sec is knocking on for the Z axis IMO. However, if you can't increase motor current, you might be able to get higher speed by reducing jerk and/or acceleration.

I am not sure if i understand the problem, but for me it sounds more like a “mechanically” problem than an "electrical" one.

When 2 steppers move the same axis and you have also 2 e.g. linear rails, any even very small distance change between the rails or other geometry parameters can “block” a carriage or stepper very quickly.
Or if one steppers is quicker than the other it tries to twist the rails, which wont work. On big CNC mills with very stiff mechanics this is a common problem.

What do you use as guides and are you able to increase the mechanical clearance or reduce stiffness ?

I am not sure if i understand the problem, but for me it sounds more like a “mechanically” problem than an "electrical" one.

When 2 steppers move the same axis and you have also 2 e.g. linear rails, any even very small distance change between the rails or other geometry parameters can “block” a carriage or stepper very quickly.
Or if one steppers is quicker than the other it tries to twist the rails, which wont work. On big CNC mills with very stiff mechanics this is a common problem.

What do you use as guides and are you able to increase the mechanical clearance or reduce stiffness ?

This is my Z axis. it uses lead screws and vertical rods like an Ultimaker but I have them on both sides instead of just the rear. With no motors connected, they run free up and down with no binding. With the lead screws in place, the machine runs so free that one side can slide the build plate up and down easily. You can even get them (with no motors) 1/4" out of alignment and still move them up and down freely.

Also, at the slower speed I have set now, it transitions from top to bottom with the build plate staying flat with relation to the machine.

Edit: Just to be clear, the stall only happens on the up travel (going toward 0). Down is fine (gravity is wonderful).